Thanks to the recent gradual decrease in the production cost of solar batteries used in solar electric generating systems, many consumers utilize solar electric generating systems at home, although many of the home solar electric generating systems are cooperation type systems connected to a commercial single phase AC system supplied to residential areas. The solar battery of home solar power electric generating systems is connected in series-parallel to several dozen power generating modules with small kW output. The cooperation type solar electric generation systems require converting DC power into AC power via an inverter circuit to be used for single phase AC systems.
The publicly known solar electric generating system shown in FIG. 24 is a typical cooperation type system controlling an inverter circuit by the PWM method and having a solar battery 100 wherein a plurality of the sub-units of a plurality of power generating modules connected in series are connected in parallel, inverter circuit 101 and control device 102 controlling the inverter circuit 101 by switching. The control device 102 is provided with a voltage detection device 103 detecting the standard voltage of the AC system, amplifier 104 amplifying the detected voltage, triangular wave generator 105 and PWM control part 106. As shown in FIG. 24, the PWN control part 106 controls the switch elements of the inverter circuit 101 based on sine waves 107 of the command voltage according to the standard voltage shown in FIG. 25 and carriers 108 generated in the triangular generator 105, and then accomplishes output to the AC system after converting to sine wave AC power by the generating square wave AC voltage 109, as shown in FIG. 25, and flattening the square wave AC voltage 109 via a filter circuit.
However, regarding the technology controlling the inverter circuit using the above PWM method, there is a problem that approximately only 90% of the output of solar battery is utilized as the output of the solar battery, and needs to be intermittently shut off to convert to AC power. Furthermore, due to the generation of harmonic components from frequent switching of switching elements in the inverter circuit and the impedance of the AC system, a large filter means or electromagnetic interference removal means needs to be provided to absorb the harmonic components. In addition, there is another problem of having a large loss in power devices, such as an inverter circuit and switching elements due to the large number of switching frequencies involving significant voltage changes.
In contrast, there is another proposal for a solar electric generation system utilizing a battery switching system by changing the output voltage level in multi-steps via switching the number of solar batteries out of a plurality of solar batteries to provide the output. In this power generation system, as shown in FIG. 26, for instance, four sets of solar batteries 110 capable of generating 10V, 20V, 40V and 80V DC power are provided and only a switch S1 out of switches S1, S2, S3 and S4 is turned on to output 10V DC power and the voltage of DC power can be increased/decreased stepwise in increments of 10V to switch to 20V, 30V, . . . 140V and 150V by combining the switches to be turned on. DC power is output to the AC system after being converted to AC power as shown in FIGS. 27(A) and (B) via an inverter circuit 111. In this solar electric generation system utilizing a battery switching system, in comparison with the power generation system previously described in FIG. 24, problems associated with harmonic waves or electromagnetic field interference are mitigated. However, there is the problem that the rate of utilization becomes significantly lower since all of the four solar batteries are used only for a short time to generate a peak voltage and one or a plurality of solar batteries are in the idle stage most of the time.
When any of the four sets of solar batteries 110 is blocked by buildings and the like and only partial sunlight becomes available, the power to be generated from the blocked solar battery 110 is significantly decreased to reduce the output voltage influencing the normal output of AC power. Furthermore, there is another problem that the DC voltage is not generated from any of the four sets of the solar batteries 110 during the nights when sunlight is not available, therefore, the power generation time by the solar battery 110 is limited to affecting the performance of power generation by the power generation system.
In recent years, the development of a power generation system utilizing a fuel cell system is advanced and is expected to be gradually put to practical use in the near future as a home power generation system.
The fuel cell is comprised of layered multiple single cells and is configured to output DC power by connecting a number of single cells in parallel. Since each single cell generates approximately 0.6-0.7V DC power, and there is the same problem as the solar electric generating system in a way which utilizes technology to provide DC power generated from a power generating system to an AC system by having the power generating system cooperate with a home single phase AC system.